Help with DARKFIELD MICROSCOPY

I own a LW Scientiic Observer IV (microscope that is) with brightfield lighting. There is a 'tray' for lack of a better word, where you can put colored lenses to color the light. On a microscope at school there is a similar 'tray' that has a few different color lenses and a setting that puts the scope into darkfield, my question is: Is darkfield lighting accomplished with a lens? If so, where can I get a lens that would fit my scope? Thanks in advance!!!!!!